text:
Is it possible to tow a smaller aircraft (like a LSA with a non-functioning engine) with a more powerful towplane?
The idea is to move an otherwise flyable aircraft with a bad engine from an airstrip with no facilities, to an FBO with a AP mechanic available for an engine swap. Let's say perhaps 100-200 miles, VFR conditions, takeoff, climb to cruise altitude, descent, and landing.

Although there are tow planes made to do this, would it be practical to tow an LSA not made specifically to be towed? Where would you attach the tow rope? How would you disconnect it in an emergency?

I don't know if you're in the US but if you are, and if "possible" includes "legal", then you'd need an FAA waiver per 91.311.

Yes.
I know of a pilot in the USA who has done exactly this.  Both aircraft were Cessna 182s.  The aircraft to be towed was outfitted with a releasable tow hook mounted on the prop shaft (I believe the prop was removed).  It took a lot of effort to satisfy the FAA that this could be done safely.  Part of this effort included videotaping towing a Cessna 182 (the one that ended up being the towplane) into the air via the same prop shaft-mounted towhook with a motorcycle on a long 7000' runway.  Since the motorcycle had much less engine power than a Cessna 182, this was evidence that the concept would work.
The end result was that the disabled plane was successfully ferried in this manner, with permission from the FAA.  I don't recall the exact starting point of the ferry flight but I believe the distance was in the neighborhood of 60 miles.
The ending point was an untowered field and as best as I recall the starting point was as well.
Naturally, the tow line was released before the aircraft entered the landing pattern.
This all happened sometime between 1990 and 2005; I don't recall the date more accurately than that.
If your LSA did not have a tractor (nose-mounted) engine, you might have more of a challenge.
A qualified glider tow pilot will be needed to fly the towplane.

As an addition. You will need a qualified glider tow pilot. Ferry flights must specify the route and avoid populated areas. There are requirements for glider tow lines and connections.

Did the motorcycle stay on the ground during the test?

@Someone-OnStrike --yes, naturally.  I don't recall how long the tow line was in that test or how high the airplane climbed -- probably just high enough to demonstrate that it was clearly out of ground effect and able to maintain altitude.  A full-blown auto-tow style glider launch using only a motorcycle obviously might be dangerous to the motorcycle driver.

Of course it is possible, but I think your main challenge will be twofold:

Rigging the LSA legally with a glider tow point that can be released if necessary.  If it wasn't designed to be towed whatever you come up with will need to be inspected and approved for flight.

Performing an intentional power-off "emergency" landing at the other end.  Without a functional engine the LSA is not airworthy, so I doubt any controlled airfield would allow you to land it there.

Not to mention how are you planning to calculate fuel needed?  Your tow aircraft performance charts won't account for the extra drag, and you aren't talking about a short trip...
I would suggest it might be easier to just go pick up the A&P and bring him back to do the swap.

...or put your LSA on a trailer and drive it there.

Re #2, if you drop the tow line, you are now in an airplane with a non-working engine, so you declare an emergency :-)  Plus LOTS of airports don't have control towers or anything, so you just listen to the Unicom (if there is one), watch for traffic, and land.

@jamesqf, right, essentially creating an emergency situation on purpose.  It would be pretty tough to explain your decision making process to the NTSB if you mess up and land short.  In my mind I'm already drafting the "Darwin Award" writeup...

You'd attach the tow rope to a bridle around the towed airplane's spinner, but, as others have noted, the risks involved are far greater than bringing the A&P and the replacement engine to that airplane.

If the airport is untowered, the landing should be no issue. It's no different than if you were landing a glider there. No need to declare an emergency. No need to consider the pilot for a Darwin Award. Obviously you are going to go to an airport with little traffic, or you are going to announce on the radio what is going on and make sure there is a break in the traffic.  Even if the field were towered, it would be no different than landing a glider there.

@quiet flyer, except that it isn't a glider, it is an aircraft with a non functioning engine that is designed to be flown with power.  Of course unpowered forced landings can be done safely, but they are considered an emergency.  A couple standard principles of risk management is to accept no uneccessary risk, and accept risk only when outweighed by benefits.  I don't think creating an intentional emergency situation is the wisest course of action in this case, but if the OP is willing to bet his pilot cert on his ability to explain his decision making process to the FAA, then go for it...

" but they are considered an emergency."-- why, when you release the towrope at a time of your choosing and a location of your choosing?  I don't think the pilot involved considered it an emergency.  You could ask, "why should anyone ever fly a glider at all, since it creates an unnecessary risk?"

Read my comment again, I’m not talking about gliders.  When an engine quits on a single engine powered plane it is an emergency.

But the engine didn't quit in flight, if it was inoperative before takeoff.

Many's the light plane that's been converted into a glider by removing the engine -- https://www.youtube.com/watch?v=JTGJcNo2LU0&feature=youtube&t=136 ( intentionally starts at t=136), https://www.youtube.com/watch?v=LC-QIEzorFI

“But the engine didn't quit in flight, if it was inoperative before takeoff.” The timeline of when the engine quit isn’t the issue here, it is the fact that you are airborne in a non-airworthy aircraft that makes it an emergency. The fact that you did it intentionally actually reinforces my point, and is what brings your judgement into question. But if you think you could walk the FAA through your rationale and convince them that this is a better option than fixing it in place then detail out the reasons in your own answer. I think it is a bad idea, & I’m not going to argue opinions with you.

P.S.  Engineered removal of the engine from a few cubs during war time by the military, sanctioned by the CAA for glider training, is pretty far removed from what this guy is planning.  In fact it ties directly into the principle of accepting risk when there is a benefit.  And it doesn’t exactly bolster your claim that “many have been converted”...


text:
Why are manufacturers assumed to be responsible in case of a crash?
I attended at a training concerning airborne software and the coach remarked that despite at the normal courts, when an aircraft crashes and the authority investigates, a manufacturer is guilty for that accident until innocence is proven that the manufactured device did not produce the defect causing the crash.
I hoped to find on the web some reference to this "guilty until proved innocent" rule, but I cannot. Do you know where I could find some info about that?

That seems odd to me, because, IIRC, most aircraft accidents are the result of pilot error.

For that I do not have any idea, but it would also have sense (the pilot is guilty until he/she proves innocence about having caused the crash). But for sure it makes definitely sense for manufactured equipment (i.e. everything may have caused the crash until the respective stakeholders show they are not responsible).

Sounds like an exaggeration of different standards of burden of proof. Regardless, you should indicate what jurisdiction you are interested in, because there might be regional differences, especially when it comes to criminal proceedings.

I don’t think you are going to find this “rule” written anywhere. As @Sanchises has stated, it is more of a proverbial statement or idiom expressing how a manufacturer is treated by the FAA (and maybe other civil aviation authorities). I don’t think it stops at the manufacturer. The pilot(s) and all other parties involved are treated similarly.

@DeanF. It is exactly what I mean; not stopping at the manufacturing but even the pilot is involved. Even as proverbial statement or idiom used by FAA, isn't there any procedure/process description reflecting somehow this behavior ?

I don’t think you will find this as a written procedure. It may be more of an aviation cultural norm. Also consider the FAA has written aviation regulations as more of a privilege than a right. Which means that the burden of proof requirements of law are not as stringent for aviation regulations. That is one reason pilots are issued certificates and not licenses. As a privilege, you are allowed to fly as long as you can prove that you can do so safely.

@nick012000: except for the two most publicised accidents in recent years - you know, the ones that led to the whole type being grounded and got the aircraft manufacturer into serious trouble? Of course, there the manufacturer also tried to frame it as pilot (or maintenance) error, that's why it needed a second crash to ground the plane.

I think some context is needed here...  Are you asking about the FAA, the NTSB, or the courts?  The question references both the "normal courts" and the "authority" that investigates.  These are distinctly different bodies with a different purpose.  One "authority" (NTSB investigating, and FAA taking action) is responsible for safety investigation with the intent of finding root cause to prevent future mishaps.  The other authority, (the courts) is trying to assign blame for financial recompense.  The (Anglosphere) courts are absolutely bound to the presumption of innocence.

@MichaelHall makes an excellent point. My assumption was that the OP was specifically asking about aviation authorities only. In which case, they don’t really have to abide strictly by the procedure of law. They only have to abide by the regulations that they, themselves, have written. They are not a court of law. In the US, a court of law has to follow the rule of law. Case in point, you can be detained and prohibited from flying simply on the suspicion of intoxication. It must be proven in court for you to be imprisoned for the offense. One Indian pilot simply handed over his certificate.

@rob74 That's because stories of things like "tiny GA plane flies into the side of a mountain after getting caught in fog and becoming lost" happen often enough that they're not newsworthy. The most heavily publicised incidents are famous specifically because they're not the norm.

"when an aircraft crashes and the authority investigates, a manufacturer is guilty for that accident until innocence is proven" – This is nonsensical for the simple reason that an air incident investigation is not a criminal trial and a criminal trial is not an air incident investigation, and the maxim "innocent until proven guilty" only applies to criminal trials.

Your average manufacturer has deeper pockets than your average pilot, hence the interest in assigning liability there.  Higher judgments equal higher fees.

I suspect the coach was talking about "the court of public opinion", meaning that many people will decide not to fly on a plane type that has had a crash, even if the reason for the crash was nothing to do with the plane.

a manufacturer is guilty for that accident until innocence is proven that the manufactured device did not produce the defect causing the crash.

No, someone has found some evidence of a "defect" after the crash and filed a lawsuit on these grounds. How sound this evidence is, and what exactly is the "defect" doesn't matter at this point: the lawsuit was filed and the manufacturer will have to defend themselves. They are still innocent until found guilty.
I put the word "defect" in quotes here because the legal definition is much broader than what people usually think. For instance, a coffee heated to 88 °C was found defective in Liebeck v. McDonald's.

I'll assume you're talking about an English common law jurisdiction (UK, US, Can, Aus, NZ etc.).  In those places it's always "innocent until proven guilty", the key thing being what "proven" means.
You have to separate criminal liability, breaking of a law in the criminal code, from civil liability (a civil wrong, or "tort" in English common law).  The criminal liability standard of evidence is "guilty beyond a reasonable doubt" at trial, and tort liability standard is "guilty based on preponderance of evidence" at trial (that is, more likely than not in the assessment of a trial jury (in the US) or a judge).
So a manufacturer or an airline is only exposed to criminal liability where an incident involved the violation of a criminal statute. You can't throw the "company" in jail, so an individual has to be on the hook and so the authorities would have to identify a criminal code violation committed by an officer or employee of the company, and it would be that employee or officer that would be going to jail (for something like violation of a criminal negligence statute).  And in that case the standard is guilty beyond a reasonable doubt.
In the case of civil liability, the corporation itself, or potentially its officers personally, are on the hook, the penalties being solely financial (employees are normally shielded from civil liability in a limited liability corporation) and the "preponderance of evidence" standard of guilt applies.
The thing is, both of these conditions require a court trial.  Either government prosecutors go after individuals (officers of the company or employees) for criminal violations that include fines and/or jail time for those convicted of criminal acts, or non-government individuals that have been harmed (families of passengers killed for example) go after the corporation in civil court, and the corporation is only guilty of a tort (civil wrong) if a judge or jury finds them guilty.
So in both situations it's "innocent until proven guilty"; the difference is in who the accuser is and what the standard of evidence is.
Now, outside of the Anglosphere, all bets are off, and you may very well find places where guilt is assumed and company has to prove its innocence to get off the hook for either criminal or civil liability.

Sure, but as also DeanF said, it is a way how FAA treats some situation strictly aside from the law.

In that case it's "politics".  Plus there are administrative professional sanctions against individuals, for violations of regulations, but that's a separate issue.

Another issue is that (at least in the US), plaintiffs will sue everyone involved, and the defendants have to foot the bill for their defense even if they’re innocent. And the cheapest and easiest defense for each defendant is to say it’s a different defendant’s fault, which also changes things.

Yeah that's "joint and several liability".  Like when they sued the maker of the vice grips that John Denver was using to operate the fuel valve of his Long Eze, plus the hardware store that sold them IIRC.  US tort law is unique.  Another one is civil jury trials, which are rare in the rest of the Anglosphere.

Corporations can and have been convicted of crimes. They can't be put in jail, but they can be forced to pay hefty fines.

Just to set things straight, accident investigation performed as per ICAO Annex 13 (Aircraft Accident and Incident Investigation) and criminal investigation regarding an aviation accident are  separate processes.
The "Annex 13" investigations aim to find out why an accident happened, so it is focussed on reason(s). The purpose of this is to prevent repeating mistakes, whatever they might be.
Criminal investigations on the other hand are usually based on a presumption someone might be guilty of, say, negligence or such, but they do certainly not presume guilt. The purpose of criminal investigation is to establish evidence of whether or not there is guilt or liability in a case.
None of the instances responsible for investigations in any civilized society make presumptions or judgement of guilt. The simply gather evindence. Now, interrogative methods might make it seem like the investigator thinks the person under investigation is guilty, but no good investigator will allow oneself to fall into that trap. Guilt or liability is established in a court of law.
It is my firm recollection that Annex 13 investigations are generally inadmissible as evidence in a court of law. The reason for this is simple: to secure the access to truthful information. If there was a chance what a whitness said to accident investigators might be used against oneself in court, they probably would prefer not to say anything. At worst, they might end up down right lying to hide their role or liability in an accident.
To be presumed guilty until proven otherwise just does not happen in civilized countries. Sad to say this does not include all nations...

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text:
What may cause a pitot tube and Honeywell pressure sensor calibration inconsistency?
It is possible that my issue is specific to my setup, but I thought I'd try here if someone has experienced a similar issue. I will try to explain this clearly as possible, but it may be slightly confusing:
I place two pitot tubes in a wind tunnel in order to calibrate the sensors they were connected to. The sensors are identical differential pressure sensors from Honeywell (model# 004NDAA5). For each point in the calibration data, I took the average voltage from a sensor over 25 seconds (without changing the windspeed). I matched this with the measurement taken by a manometer. I did this for both sensors at the same time, and for a total of 15 points each. I did not include low windspeed measurements (below 5 m/s) due to the sensors low accuracy at that range.
Next I plotted the voltage data against the pressure data. This gave a linear relationship for difference in pressure (stagnation - static). I then used the following equation:
velocity = sqrt(2*diff_pressure/density)
After I did this, the two tube/sensor pairs agreed on the windspeed. The next day, they disagreed consistently by 3-4 m/s. I'm rather stumped as to what might cause this. To add, neither agree with a hotwire anemometer connected to the tunnel.
Things I've done to troubleshoot:

I swapped the tubes with each other to see if they affected the sensor reading, they do not.
I swapped the sensors pin on my Data acquisition (DAQ) setup. This swapped the voltages as expected.
I've confirmed with a multimeter that the voltage I am reading with the DAQ is the correct voltage output by the sensor.
Confirmed that the tubes were not within the boundary layer of the tunnel

Any suggestions?

Did the positions of the pitot tubes in the tunnel change between the days?

@DeltaLima Yes, they were placed in a different tunnel entirely. But I don't see how that would have this effect. I've also confirmed that neither tube was in the boundary layer of either tunnel. Further, this effect is present no matter the tunnel.

Just looking for clarification: Are they identical pitot tubes as well? Oftentimes body effects fore and aft of the static ports can cause variations in data. This wouldn't necessarily account for why the measurements changed, but it may offer some insight nonetheless. An accurate pitot-static should track the trend of a calibrated manometer, but with a constant offset caused by variations in pitot design.

Yes, the pitot tubes are identical as well. The reason why I swapped them (as mentioned above) was to determine if there was some sort of damage/blockage in one of the tubes. However,  swapping the tubes had no effect on the voltage output by the sensor.

Boundary layers of the tunnel or aerodynamic interference between the two bodies of the tubes / mounting constructions were what I had in mind.

Did you do a static pressure check of both the static and the stagnation pressure sensors?

I've never tried to calibrate a differential pressure sensor with a wind tunnel, usually use a Laversab. I'm not sure about those particular Honeywell sensors, but the ones I am familiar with tend be quite noisy at lower airspeeds (< 20 KTS)

Is it a new method? Meaning, has the method been used in the past and proven to be consistently correct?

@DeltaLima I'm not sure what you mean by static check. Also for clarification, there are two DIFFERENTIAL pressure sensors. So there isn't one sensor gathering just static P and the other stagnation P. Both sensors measure the difference between stagnation P and static P.

@Koyovis Yes, this has been done many times.

Is there any hysterisis in any of the system variables at all?

Pitot are sensitive to dirts, please try to Add a new tube, thus having 3 tubes, as is done in modern aircrafts , the trouble is from one of the 2 tubes you have used. The 3 tubes method will let you find the damaged one or dirty one.....they are very sensitive to dust, dirt etc

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text:
What control inputs are required to perform a "turning-while-in-flight" maneuver in a helicopter?
My question is relatively simple and has to do with the required control inputs to perform a simple turning maneuver over a large area.
Suppose a helicopter is traveling in forward flight, straight and level, and the pilot has to alter course to circle a large area. Now, suppose the helicopter begins a slight rightward turn, while continuing to fly 'forward', and ends up semi-circling over a broad area.
I want to make the point that the helicopter is not yawing/turning on its own axis, but rather performing a turn over a large area.
The resulting flight path, if viewed from above, would essentially be a semi-circle or horseshoe shape.
This would obviously involve some rightward banking. What I want to know is this: can this maneuver be performed with only cyclic inputs, or would there also have to be pedal inputs to complete such a turn? And if it can be performed only with cyclic inputs, what exactly would those inputs be apart from rightward cyclic? Thanks in advance!

Perhaps you mean a coordinated turn?

I don't quite understand your description. Suppose that, at the beginning of the maneuver, the nose of the helicopter is pointing north. After it flies a semicircle, is the nose still pointing north (meaning that for the last part of the maneuver, it's flying backwards instead of forwards), or is it now pointing south (meaning that the helicopter must have yawed during the maneuver)? From your description, it sounds like you're saying that as the helicopter flies in a semicircle, it never flies backwards and never yaws, which is practically impossible.

I find the case where a helicopter performs a turn largely by yawing and then braking with cyclic much more interesting... of course the wind moving across the body of the helicopter make this harder to do.

@TerranSwett -- re "I want to make the point that the helicopter is not yawing/turning on its own axis"-- I think what he really meant was that the turn is not centered around the aircraft's own vertical axis.  Not that the helicopter's yaw rate is zero.  There is some room for further clarification here, but I think he's basically describing a normal coordinated turn.

(Presumably meaning the yaw string is centered, which in helicopters, may mean that the ball is a little off center, due to the side thrust generated by the tail rotor?)

Yes it's cyclic inputs only for banked turns because there is no adverse yaw due to ailerons to compensate for.  You will be working the pedals to keep the ball centered, but this is only in reaction to torque changes from increases in rotor pitch and power demands.
Note that the airplane isn't using rudder to turn either; only to correct for adverse yaw.  What brings the nose around in the turn is the vertical fin.  The banked wings creates a lateral force vector, moving the airplane sideways as it goes forward.  The sideway motion cause the airstream to strike the vertical fin from the side, and the airplane's inherent weathervaning tendency brings the nose around to keep it aligned with the arc of a turn.  If an airplane had no adverse yaw at all, you would not need to use any rudder to help with turns.
A helicopter is the same.  There is a small fin back there that gives it modest weathervaning tendency (the disk area of the tail rotor also contributes).  The pedals are just there to change tail rotor pitch to adjust for off-axis yawing motions induced by torque changes (which can be in either direction, depending on whether torque is going up or down); just as on the airplane the pedals are just there to compensate for adverse yaw due to aileron displacement. If the helicopter had some automated system to always compensate for torque changes, you would not need to touch the pedals to turn.
In practical terms, in both cases, when flying you simply do whatever you have to do with your feet to keep the skid ball centered; in the airplane you are generally reacting to aileron movements, and in the helicopter to torque/pitch changes.  The power plane pilot learning helicopters has to adjust to get out of the habit of squeezing a little into-turn rudder to anticipate adverse yaw, and has to instead anticipate the subtle yaw changes - in either direction - from the power and pitch changes the pilot may input during the turn.

While small and due to other phenomena, there is adverse yaw in a helicopter.  Typically all four controls are moved to turn and maintain altitude & speed.

Yes there are yawing effects, but not adverse yaw in the aileron sense which was my main point.

As usual in a helicopter, you typically need to adjust all controls because everything is cross-coupled.
The primary input to initiate a turn is lateral cyclic. This banks the helicopter and tilts the lift vector so that a centripetal force is formed.
The pedals are only used to make small corrections and keep the helicopter aligned with the incoming airflow. Usually a yaw string attached to the outside of the cockpit window shows whether the helicopter is in a skid, slid or coordinated turn.
Since the lift vector is tilted due to the banking, additional lift will be needed to maintain altitude. For this, collective and throttle are increased. Again, this has an effect on the torque, so the pedals are needed to keep the nose pointed into the airflow.
For steep turns, forward cyclic may be required to keep the airspeed from dropping.
From the FAA Helicopter Flying Handbook, chapter 9:

How fast the helicopter banks depends on how much lateral  cyclic
pressure is applied. How far the helicopter banks (the  steepness of
the bank) depends on how long the cyclic is  displaced. After
establishing the proper bank angle, return  the cyclic toward the
neutral position. When the bank is  established, returning the cyclic
to neutral (or holding it  inclined relative to the horizon) will
maintain the helicopter  at that bank angle. Increase the collective
and throttle to  maintain altitude and rpm. As the torque increases,
increase  the proper antitorque pedal pressure to maintain
longitudinal  trim. Depending on the degree of bank, additional
forward  cyclic pressure may be required to maintain airspeed


text:
What type of antenna is this?
I’m just getting more into airplanes and trying to do a project on antennas. This airplane example is a Piper PA-28-181 Archer II. I’ve labeled all the antennas, and I’ve noticed there’s a lot!
I’m specifically curious about #1. I know it looks more like a com antenna, but it also doesn’t look like most antennas I’ve come across. Any insight on why antenna #1 looks different than other com antennas and/or if there’s a more specific purpose for it?
Also, I’d love to learn more, so if someone is willing to label what each one does to confirm what I’m researching, I’d really appreciate it. Thanks!

Welcome to aviation.SE! This question might be helpful.

Its a comm antenna, Belly mounted antenna have a few purposes

Worth mentioning is properly planning the antenna placement for modern
audio panels, especially if you are installing antennas before you get
into the audio panel wiring. Both Garmin and PS Engineering panels
have a split-com mode. This is where the pilot can transmit on one
radio and the copilot on the other simultaneously. While most com
radios should be wired for transmit interlock (another reason to
carefully follow the wiring schematics), for the split mode to work,
one antenna should be mounted on top of the aircraft and the other on
the bottom. That should be enough separation for closely tuned
frequencies on both radios.

Since you are flying and generally controllers/towers/centers etc are below you it can be advantageous to have the antenna on the bottom of the aircraft so it's in a direct line to the receiver.

Note: I'm not exactly an expert in this, so it's possible I've made some mistakes. I'm sure someone in the comments will correct me if that's the case.
#1: As the other answer says, this is the antenna for one of the two-way VHF radios. It's on the bottom because airplanes are typically above the ATC facility they're talking to, and so putting the antenna on the bottom may give it a better line of sight.
#2: This is the antenna for the marker beacons, part of ILS.
#3: This is the UHF antenna, used for both the transponder and DME (if installed).
#4: This is one of two navigation antennas (the other one is on the other side of the tail, not visible in this photo). They're mounted at a 45 degree angle to the aircraft centerline, and are typically used to navigate via VORs. They may also feed other navigational equipment.
#5: I'm not sure about this one. I think it might be the ELT antenna.
#6: This is the antenna for the other two-way VHF radio. It's mounted on top on the theory that, if you can't get a good signal on one radio, another radio with an antenna in a different position might give you better reception.
#7, #8, and #9: These are all satellite receivers for stuff like GPS and satellite weather, though you can't tell from the outside which is which.


text:
Do dark conditions due to high-altitude smoke prevent ultralights from flying?
According to FAR103, you have to operate ultralights between sunrise and sunset, or between the times laid out in the Air Almanac (if you're in Alaska.) This means that the FAA has determined that certain lighting requirements exist for ultralight pilots, but has laid them down in a time-defined manner, instead of a lighting-involved one. Under normal situations, this is fine - you don't get enough eclipses, volcanic eruptions, or alien invasion fleets blocking the sun to make rules based upon luminosity.
At the moment, there are a number of arguments breaking out about whether Ultralights can fly in my area. It's just after noon, but because of the fires in California, high-altitude smoke and clouds are bringing lighting conditions to being comparable to 30 minutes post-sunset.
If the sun is up and pilots stay clear of clouds (and smoke plumes), and - of course - we're not violating a TFR, is the assumption that we can fly without a 3-mile visible light, or is there something in the FAA regs that deal with this situation?
To reiterate - we're not flying anywhere near the fires, we're not flying in a TFR zone or other controlled airspace, and we're not flying within a mile of a cloud or smoke plume.
Thanks...

The only regulations that apply to ultralights are in part 103. Apart from the obvious visibility and cloud clearance (103.23) and daylight (103.11) ones I see two others that could apply:
103.9(a):

No person may operate any ultralight vehicle in a manner that creates
a hazard to other persons or property.

This seems to be the part 103 equivalent of 91.13 (careless and reckless operations) and it's also written broadly. If you do go flying and something goes wrong, the FAA might argue that flying in marginal conditions was creating a hazard to other pilots or - if you lost control of the aircraft - to people on the ground. Whether or not that's plausible would depend heavily on the specific situation, of course.
103.21:

No person may operate an ultralight vehicle except by visual reference
with the surface.

In low light conditions with a lot of cloud and smoke around, it could be difficult maintaining visual reference.
So, if you're flying at noon, maintaining the required flight visibility, maintaining visual reference with the surface, staying out of TFRs, and not doing anything dangerous, your flight is probably legal.
But the important question here is "is it safe?", not "is it legal?".


text:
Can I do a VFR night sightseeing flight in Europe?
I am an American living in the Netherlands, and I recently converted my US PPL to an EASA PPL.  I have significant night flying experience in the US, but I understand that night VFR in Europe is uncommon, and in fact only this year became authorized in NL.  I know a flight plan in required, but I'm unsure whether I can file a plan with departure and destination the same, then just go fly around a bit and return at the specified time.  Or must I have an actual route to a destination that I have to adhere to?

Does your PPL give you privileges to fly at night? They don't come with that automatically, you have to get a night rating.

@GdD I have just applied for it based on credit received from an ATO for my US experience, so I will have it shortly.

Why would you not be able to file a round robin flight plan? It is common in the US to do so for instrument training. In areas with frequent sightseeing flights, there is probably even a common route. TRACON in our area calls it the downtown tour. You can put it as such in the notes of your flight plan. Once airborne, you can request to divert as necessary.

"Why would you not be able to file a round robin flight plan?" there could be regulations limiting night VFR. OP's question is a good one.

related: https://aviation.stackexchange.com/q/16488/1467

@DeanF. basically I am asking this exact question. Procedures in Europe are quite different from the US, and in fact instrument training is accomplished quite differently as well (by booking a "slot" ahead of time at the desired airfield, for example).  I could just try to file it "round robin", but I'm interested in the legalities & customs, which I cannot find anywhere, probably because it's VERY uncommon.

Yes you can file a flightplan with the same departure and destination. You generally do not need to include detailed routing, unless local rules require you to do so (but if you are planning to cross a country border, you probably need to include a point of crossing and estimated time). One thing to consider is that the EET in your flightplan should be accurate, since alerting service is based on that (if you don't arrive within a certain amount of minutes after your ETA, search and rescue efforts will be initiated). However, as long as you are contact with ATC while in the air, you can always just inform them of a revised ETA, and they can pass that on to your destination airfield.


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Why does the B-52 take off without rotating?
In the answer to this question it is noted that the B-52 takes off without rotating and climbs out in a nose-down attitude. Why was it designed this way?

as far as i know the B52 makes use of ground effect and the setup of the undercarriage wont allow for rotation. if it did the pilot would have to (evan for a moment) be on one set of wheels

@chaos505: is there an ELI5 on rotation?

Take a look at this video, https://youtu.be/CCfJmuk-des Lots of B52s rotating at takeoff. Nosewheel comes off first, then first set of wheels, then second. Compare the takeoff to a B29 https://youtu.be/rcvDQXMh-DM. Pretty similar attitudes. So to say that they don’t rotate is wrong. Obviously, not the same as a C5 Galaxy https://youtu.be/eCC6ZbZvG8I but they have completely different missions.

@JScarry Yeah, most of them do have a very slight bit of rotation, but nothing like you see in other aircraft. Check out this article from a pilot talking about how, instead of rotating, they "unstick." Basically they keep it as level as possible, but they don't want to wheelbarrow. I've seen some videos where there is no discernible rotation. Front and back leave the ground pretty much simultaneously.

@JScarry: The change in pitch attitude is from the climb path and not from increasing lift for liftoff. Basically, the airplane changes from horizontal acceleration to a combination of climb and acceleration, the climb causing it to change pitch attitude. No rotation to see here - just watch the elevators which don't move (which they would have to do in order to command a rotation).

The reason was to give the bombs the place close to the center of gravity.
Wing sweep (for high cruise Mach numbers) in combination with a high aspect ratio of the wing (for low induced drag) made it impossible to place the landing gear in the wing, so it had to be integrated into the fuselage. The main landing gear normally needs to be close to the center of gravity (slightly behind for a tricycle gear, slightly ahead for a taildragger), but this space was needed for the massive bomb bay. Since the bombs will be dropped somewhere along a bombing mission, dropping them should not upset the balance of the aircraft, so no compromise was possible.

B-52 cutaway drawing showing the two bomb bays in the center of the fuselage (picture source)
This problem existed already during the development of the 6-engined B-47 a few years before. In both designs, two pairs of landing gears were chosen, one pair ahead and one pair aft of the bomb bay, and the aircraft lost its ability to rotate for takeoff. Due to the powerful fowler flaps the attitude in cruise and at slow speed could be made identical. The downside is more drag during the take-off run, since the wing produces more lift, but this could be tolerated in a strategic bomber with air refueling capabilities.

B-47 on approach with gear down and drag chute deployed (picture source). Note the outriggers between the inner pair of engines - those were needed to keep the aircraft level on the ground.

B-52 in flight with gear down (picture source). Here the forward and aft gear of the B-47 have been replaced by pairs of gears to distribute the load over eight wheels and the outriggers are positioned outside of the outer engine pair, but the general gear configuration is fairly similar.
The Russian design bureau Myasishchyev found a different solution for their M-50 supersonic bomber in the mid-50s. They also had to put the bomb bay into the center fuselage and the main gear had to be placed so far aft that the M-50 could not be rotated the usual way with the elevator. To solve the problem, the engineers devised what they called the "galloping bicycle". When the aircraft reached 300 km/h, the forward gear rapidly extended to rotate it to 10°.

Myasishchyev M-50 with extended front gear strut (picture source). The two open doors under the cockpit were for the pilot and navigator: Their downward-ejecting seats would be lowered on cables for the crew to be strapped in at ground level, then winching themselves into place.

When you say rotation are you talking in terms of pitch?

@DavidGrinberg: Yes. This is a fixed term describing the pitch motion from level to flight attitude during take-off.

FYI, the B-52 only has one bomb bay. You might also mention the relatively high incidence angle. Further, the B-52 does rotate, just not to the level people are used to.

"Wing sweep (for high cruise Mach numbers) in combination with a high aspect ratio of the wing (for low induced drag) made it impossible to place the landing gear in the wing" - the same applies to all commercial jetliners, and they seem to manage just fine with wing-mounted landing gear.

@Sean: No, not with a high wing, high aspect ratio (8.6) and wing sweep (37°) and no increased root chord. Easy loading of and ample volume for bombs close to the center of gravity demand a high wing and leave no space for the landing gear. The wing root is too far forward of the center of gravity, so there is simply no space for a conventional gear.

@PeterKämpf: You still haven't explained how all this prevents the use of wing-mounted landing gear.  Looking at your first image, it looks like it should be possible to put the main gear just behind the center of mass by mounting it to the wings just inboard of the inboard engine nacelles and just forward of the flaps.

@Sean: Yes, in theory a landing gear at that location would work. Only that for retraction it would need a large volume which the wing alone does not provide, so there would be a need for Tu-154-like pods. Next, the mass of such a landing gear on a high-wing plane would be prohibitive. The chosen solution is much lighter, and that it requires longer runways was considered acceptable.

@Vikki- The landing gear cannot be mounted in the wing. There isn't room, and they would get in the way of weapon loading. Also, main gear in the location you specified is too far forward, in line with the CG or too close to it, in any measure.

@ Peter Kämpf You are correct in describing the design of the B-52s landing gear. However, the orientation of the B-52s landing gear has little to do with its odd characteristic of wanting to climb out nose-down. That is primarily due to the fairly extreme angle of incidence of the wings. Even if it had (somehow) traditional tricycle gear, that angle of incidence would force the nose low. This I know from experience flying in B-52Gs. When Vr is reached, without positive input, the rear main gear leave the ground first. Even when they don't, it still wants to climb nose down.

@SaxonXander: The incidence was chosen to keep the fuselage horizontal while flying above 50.000 ft and to allow taking off without rotation. That the B-52 cannot rotate is exactly due to its landing gear and forces it to use this high incidence. It doesn't want to climb nose-down, it is forced to climb nose-down.

@Peter Kämpf I'll grant that 'forced to' and 'wants to' are exactly the same thing, but the angle of incidence is what forces it, not the landing gear. The U2 has bicycle landing gear. By your definition, it should take off without rotating and climb nose-low. It does not, ergo, the landing gear has little to do with the characteristic described. Also, B-1s and B-2s rotate like normal aircraft, and both carry similar weapon loads. Taking off without rotating is not a necessary bomber characteristic.

@SaxonXander You don't understand. The issue is the location of the wheels: The one o the U-2 is exactly where it is on most other airplanes: Right aft of the cg, so rotation needs only a little downforce on the tail. On the B-52 it is so far back that no amount of downforce would lift the nose gear off, ever. Only wing lift will do that, and to produce this lift the wing needs its incidence. Taking off without rotating is forced by the wheel location on the B-52.

Adding to the excellent Peter's answer who explained why for this particular model the wheels are placed far behind the centre of gravity (CG), I would like to clarify why this makes it impossible to rotate at take off.
A standard aircraft takes off right after the rotation, increasing the angle of attack and the lift. Before and while performing the rotation the lift produced by the wings is not enough to raise the position of the CG. Still, with the wheels placed right behind the CG, a small raise of the CG is required during the rotation. This is accomplished with the down force produced by the elevator and its big leverage.

If the wheels are moved backwards, then such lever becomes much less advantageous: the force produced by the tail on the CG is weaker, because the fulcrum is closer to it and farther from the CG. The maximum elevator down force and the structural strength can then make the rotation impossible.

The angle of attack of the wings of a B-52 is positive. The leading edge of the wing is higher than the trailing edge. So when you see the aircraft takeoff it seems to not rotate, but when the fuselage is level, the wings are at a positive angle of attack. When the B-52 is in level flight, the nose is down, you can not see the nose from the cockpit and it is like you are sitting on a cloud.

Great answer that gets immediately to the crux of the issue - how it can takeoff without rotating very much at all.

Yes, but your terminology is off a little. AoA is regulated by the elevator. The angle of incidence is the angle of the airfoils chord to the fuselage. Most airliners have a 2° or 3° nose up angle of incidence. The B-52 has an 8° nose down angle of incidence.

@SaxonXander -- when you say "nose up", are you referring to the "nose" of the airfoil, or the actual nose of the plane?  The normal terminology is to say that the wing has "positive incidence" if the mean chord line of the airfoil is higher in front, and lower in back, compared to longitudinal axis of the fuselage.  By this terminology, the B-52 wing definitely has strongly positive incidence.  I can't tell whether you are saying that (by this terminology) the wings of airliners would be said to have negative incidence, or just much less positive incidence than the wing of the B-52.

(The latter situation is the actual one.)

The B52 does "rotate" on takeoff just not to the degree of what seems normal for such a large jet.
All airplanes must produce lift that is greater than the opposing force of gravity/its weight for it to leave the runway. When lift = weight/gravity the airplane is in a stable state, which means the rate of climb or descent will be constant. This means unless the lift exceeds the weight an airplane will never leave the ground. Once in flight the airplane's rate of climb stabilizes, or becomes constant when lift = gravity/weight (the opposing force).
The answer is it only appears the B52 is not rotating. The pilots are applying "up" elevator and increasing the angle of attack of the wing, thus increasing lift.
I will throw a correction to an answer given above:
The CG, center of gravity, is fixed and unless a weight/load moves within the airplane it never changes. Examples of things which would change an airplanes CG are fuel burn off, bomb dropped, or a cargo shift.
When the elevator is moved the airplane "pitches" up, because the tail (horizontal) stabilizer is producing "negative" lift (it pushes down on the tail), and rotates the airplane along the CG. The placement of the landing gear actual opposes the rotation of the airplane in most designs. The change in "angle of attack" of the wing due to the pitching up increases the lift produced by the wing.
The placement of the landing gear is a compromise based on the design of the airplane. The B52 and its tandem wheel set is a compromise because of the wing and body design of the airplane.
Add one more thing to prevent confusion:
When an airplane "rotates" for takeoff the force (or weight) of the horizontal stabilizer must be greater than the weight of the airplane in front of the rear most wheels for the airplane to "pitch up."
Most (if not all) large transport type large airplanes the whole horizontal stabilizer (compared to just the elevator on smaller airplanes) is trimmed for take off to provide a "neutral" force for the desired takeoff speed. If the stabilizer trim is set incorrectly there may not be enough "elevator" to either pitch up or to prevent a spontaneous un-commanded pitch up of the airplane. Both are disastrous and the results of which can be found on youtube.

Can you explain how when lift=gravity the a/c can be climbing? It seems to me if lift= gravity then you would not climb or descend, regardless of attitude. And actually, if you are nose-up the lift factor would tilt aft just a bit (depending on wing design) and lift would have to be greater than gravity to maintain the same altitude.

First to understand the basic of aerodynamics the wing doesn't care where it is pointed, it only knows the "relative" wind and angle of attack. The reason an airplane climbs when lift = gravity/weight is because it has more thrust than it needs to maintain a constant airspeed.   The lift does move as the wing changes speed and or shape, this is known as the "center of pressure." As the angle of attack changes the center of pressure will move and that is a function of the design of the airfoil.

On second thought, I think I get it. Due to inertia, if lift= gravity and thrust= drag then the vector of the aircraft will be unchanged, whether that be climbing or level or descending. Am I thinking right now?

Tom: Yes when all four forces are equal the airplane is in a constant state. This means in level flight the speed is constant, altitude does not change. In a climb or descent it means the rate of climb or descent is constant and the speed is also constant. Anytime one of the forces changes something else changes.

Am I correct about the lift vector tilting backward with the pitch. I get confused because sometimes lift seems to be defined as force opposing gravity and sometimes as force perpendicular to chord.

Tom: Again the lift vector is known as the "center of pressure" and it does move as the lift/wing changes. How much and in what direction is  a function of design and angle of attack. Remember the pitch of the airplane effect the angle of attack, but a positive pitch (relative to earth)  doesn't always mean a positive angle of attack. Here's the formula for low speed wings with a constant shape: ac = c/4 where ac = aerodynamic center and c = wing chord. Another formula is this: cp = (S[x * p(x)]dx) / (S[p(x)]dx)  cp = center of pressure.

Go to this web page from NASA, it provides a better technical explanation:http://www.grc.nasa.gov/WWW/K-12/airplane/cp.html

Thanks for all the explanation. As for the B-52, watching this video if it rotates at all it is imperceptible. Both front and back gear appear to lift off at exactly the same time. It's hard to tell real precisely but after pausing and rewinding several times I can't find a place where front gear appear to be up and back gear on the ground,

When an airplane "rotates" for takeoff the force (or weight) of the horizontal stabilizer must be greater than the weight of the airplane in front of the rear most wheels for the airplane to "pitch up." This is not true. The h-stabs have a huge moment arm. The net pitching moment just needs to be positive, which means roughly that the integral of the total downforce (actual weight + aerodynamic forces) over the distance from the mains to the tail needs to be greater than the same integral from the mains to the nose.

The angle of incidence on the B-52s wing is very steep, at 8° nose down. This is compared in this example to airliners, which have a 2° or 3° nose up angle of incidence. If left alone, when airspeed increases enough to lift the aircraft, the rear landing gear will lift off first. When I flew the simulator, (and I noticed this on subsequent flights) the tail of the airplane would sort of 'wiggle'. Aside from airspeed, this was an indication to apply elevator to bring the nose up. Most of the time, all 4 main gear lift of at the same time. Sometimes the forward gear would come up first (but not by a lot). But since the angle of incidence is so high, the aircraft wants to fly nose down, which is backward of 'normal' aircraft.

I may have missed it, but it seems none of the answers addresses the main issue: why it does not rotate and climbs out in a nose-down attitude?
All of this will be clearer if you remember that lift depends, mainly, on angle of attack and speed.
The main way for  any conventional airplane to take-off (and land) without rotating in pitch is by making the angle of incidence of the wings (approximately the fixed angle with which the wing attaches to the fuselage)  equal to the take-of angle of attack with the aircraft parked on the ground. This way when the B-52 reaches its design lift-off speed it will takeoff in the same parked attitude.
AS it accelerates it has to reduce the angle of attack from max lift to climb lift, which is lower due to higher speed and the upwards help of the engines,so it has to pitch down.
When it reaches its cruise speed, it needs a very small angle of attack for the same lift and, to reach it, the nose must be pitched down even more.
This increases the drag of the fuselage, tail etc.; but is part of the compromise made to avoid rotating on take-off (and landing).
At max speed the B-52 flies with the nose markedly down, which can be seen when it flies in formation with faster aircraft.
It must be noted that long-range aircraft normally set the wing's angle of incidence close to their optimum cruise speed angle of attack, which is very low compared to their max lift angle of attack. These angles all vary with weight, other non-standard design variables. The goal is to cruise with the fuselage aligned with the relative wind (from the front) to reduce drag.
BTW, glider pilots practically also do not rotate during take-offs and get their take-off lift from the speed of the tow plane.
I've never seen a source for this explanation; but this is the result of an analysis based on my experiences as an aircraft designer and pilot. Tried to make it a bit less complicated than it is, rigorously speaking..

But, don't most airliners also cruise at their max L/D speed?  Just like the B-52, right?  So if their wings are also set at on the fuselage at the optimal angle for such a situation, shouldn't they too be capable of no-rotation takeoffs?

Most airplanes designed for long cruise flights, have wings installed at cruise angle. This is a small AoA. Lift depends on AoA X Speed. Since lift during cruise has to be constant - so the plane flies horizontal - when you increase speed for cruise, you must reduce the AoA from the take-off high to the cruise low. With the the cruise angle you position the fuselage so that aligned exactly with the wind passing the aircraft to minimize drag. If all is kept constant, the aircraft gets lighter due to the burnt fuel, and the lift will make it climb. Airlines accept that. It has advantages

So why didn't they just make the front gear legs of the B-52 longer, so that fuselage sat a little nose-high on the ground, and could be totally horizontal in cruise? This would still permit no-rotation takeoffs--the angle-of-attack of the wing with all the wheels on the ground could be exactly the same as it is with the present situation, but the incidence angle between the wing and the fuselage could be much less, to optimize for cruising flight rather than takeoff.  Might make a good new ASE question.  PS didn't the B-47 fuselage sit nose-high on the ground, just as is being suggested here?

I thought I have witnessed - and a B-52 navigator (Maj, USAF) once told me - that the tail wheels actually come up off the ground first. Or at least will, if left alone with no control input, I forget exactly what he said. But basically unlike any other aircraft, the placement of the tandem gears and the super large horizontal tail allow for a lifting moment of the rear wheels off the ground before the fronts can be lifted off, if care is not taken.

Yes, you heard that correctly, but the reason is the angle of incidence of the wing, mostly. The giant horizontal stab certainly helps.

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How are antennas integrated into stealth aircraft?
In Hephaestus Aetnaean's answer to this question, it is mentioned that the F-117 had no radar detection equipment because at the time it was developed the designers had not figured out how to integrate the antennas into the body without increasing its radar profile. How is this done on newer stealth aircraft?
And what about things like air speed sensors? Do they use something other than a pitot tube?
As a related question, obviously if this was the case they apparently were not able to place radar detection inside the cockpit as you can with police radar detectors. Is the cockpit glass in the F-117 somehow opaque to radar or is there another reason?

The question did not say the F-117 did not have radar detection, it said it didn't carry a radar. The F-117 does have radar detection for missile warning.

@GdD this answer does indeed say The F-117 doesn't have a radar warning receiver

Maybe of interest, in this video an F-35 pilot mentions that it has a fixed radar antenna array instead of an antenna that sweeps back and forth due to the movable antenna giving a larger radar return. Other interesting information about stealth in the video as well. It doesn't answer this question, though.

I think the answer is wrong in that way @fooot. In the book "Skunk Works" an F-117 pilot writes about being over Baghdad in the first Gulf War, and he wrote about radar warning when a missile locked onto him because of a stuck bomb door.

Air speed is calculated using a pitot plate instead of pitot tube to help reduce radar signature.

@GdD - Does the F-117 have a RWR? It's always been a contentious point. Some say yes, some say no, some 'I like my job.' In a lengthy 2007 interview, Lt. Col. Dale Zelko (the F-117 pilot shot down) declined to reveal whether the RWR (if any) warned him of the missile launch, citing sensitivity. He only reconfirms that he visually tracked the missiles. My guess is the F-117 may have a limited radar warning capability, but perhaps not in the relevant band. If he or the mission planners had been aware of of the radar sites, you'd expect them to plan better routes to avoid the threats.

@HephaestusAetnaean I read that interview from your link on the other question. Absolutely fascinating. You're right, he did very specifically decline to answer whether he had an RWR or not.

Pitot tubes
F-117 pitot tubes

Source: YouTube. Also a look at the FLIR and DLIR.
F-22 pitot tubes

Source: AIR-SCENE UK.

Incorporation of those pitot probes were a cost savings measure. They are also a dominant RCS signature source in the forward and side sectors. The original ATF proposal was for a flush air data system that would have cost many $ millions. - djcross, F-16.net

F-35 pitot tubes

Breaking Defense. More pics on F-16.net.
Canopies
The canopy keeps radar energy out of the cockpit, the inside of which is filled with messy odds and ends. The F-22 and F-35 canopies are gold-impregnated for this reason. Note: the F-22's canopy is possibly doped with indium-tin-oxide, not gold.
In the EA-6B's case, the gold keeps the a/c's own radar energy out of the cockpit---those ram-air turbines are powering two jamming pods. The coating (and a similar one applied to visors) also turn black to protect the pilot's vision from nuclear detonations.

Source: USAF

Source: Wikipedia.

Source: Lockheed Martin. Flickr set.
Notice the similarly tinted/treated window beneath the F-35's nose. That's the aperture for the EOTS (electro-optical targeting system), an IRST and ground targeting system. Also note the two small pitot tubes above the EOTS. I remember pitot plates were being developed for the F-35 (which are stealthier but more difficult to develop), but I haven't heard anything about it in a while.
Sensor apertures
The coverings are transparent to the operating bands of the radar/radio/etc underneath.
The F-22 and F-35 have fixed AESA radars. The latter is angled up to minimize returns.

Credit: Northrop Grumman. Hi res.
Some radar energy enters through the aperture. Whatever isn't reflected is absorbed by the radar itself or heaps of RAM, which hides the messy rear/sides.

via F-16.net. More AESA pics.

Fun note, the first F-35 radar pic is of an APG-81 installed on the CATBIRD:

Source: YouTube

Did the F-117 have a radar warning receiver?
It's always been a contentious point. Some say yes, some say no, some say 'I like my job.' In a lengthy 2007 interview, Lt. Col. Dale Zelko (the F-117 pilot shot down) declined to reveal whether the RWR (if any) warned him of the missile launch, citing sensitivity. He only reconfirms that he visually tracked the missiles. My guess is the F-117 may have a limited radar warning capability, but perhaps not in the relevant band. If he or the mission planners had detected and thus been aware of of the radar sites, you'd expect them to plan better routes to avoid the threats.
I leave you with a quote from Maj. Gen. Carlsons' (who flew the F-117) briefing on stealth fighters in 1999, shortly after an F-117 was shot down in Allied Force.

We started out a long time ago building airplanes that had low observable technology incorporated into their design. The SR-71 was an example of where we took the aerodynamic design and then added some radar absorbing material to the airplane to make it slightly stealthy. And I'll give you a representation of that graphically here in a minute. We went to the second generation of airplanes and you can see we designed that airplane, the F-117, essentially from the bottom up to be stealthy. It was crude technology. It was developed at a time when we didn't have the modeling and computer power we needed to make the kind of aerodynamic design that we would have liked, but we built one that we thought was very stealthy. And of course, the night that Desert Storm opened the quote from Col. Al Whitley still is famous in the Air Force: "Boy, I hope this stuff really works." And of course, you know that it did. That isn't exactly what he said.
Then we came to the third generation of stealth airplanes. We built the B-2. And of course, by that time, we had the modeling tools and the design tools and the computing power to make an aerodynamic design that was optimum. And this airplane is [a] much higher altitude, much better performing airplane than the F-117. We were able to eliminate a lot of the radar absorbing material from the structure. And by the time we got to the fourth generation, we were able to add supersonic speed, the agility of an F-15, F-16 class airplane and do that with no degradation to the stealth. In addition to that, we were able to add a number of apertures, in other words, openings in the airplane's surface for antennas, radars and other sensors. And in the F-22, as an example, there are over a hundred of those apertures on the airplane, where if we jump back a couple of generations to the F-117, there are essentially a couple of aperture openings and the rest of them we hide when we go into combat. [Emphasis mine.]

Some of the F-22's apertures...

Source
...whose functionalities were consolidated in the F-35's fewer apertures (some shown here).

The F-35's ASQ-239 EW suite, an evolution of the F-22's ALR-94, consolidates 30 senors down to 10.

[Apologies, pressed for time. This answer will be brief, unlike my previous 29,999 character response.]

This answer will be brief Very thorough answer. I had never noticed the gold tinted windows before. Very cool.


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How do airlines bring spare parts to aircraft stranded outside their main facilities?
No professional relationship with aviation. Just an avid follower. I guess it depends on location/part availability, etc, but looking for what typically happens. For the purpose of the question, aircraft can not fly without repairs.
I see many instances where certain parts of the aircraft need to be replaced like this UPS instance where aircraft burst 4 tires and was stranded for many hours.
I don't know, but most likely aircraft is stuck in this scenario because new set of tires are not available or lack of repair facilities. On the other hand, I have read many incidents on same site where tires were replaced and aircraft was on its way in couple of hours.
How do airlines manage scenarios in such cases? For example, will the UPS fly in parts from their main facility or there are part distribution centers available across globe and they can order the parts from these centers? Can such parts be rented? Do airlines rent/buy parts from each other?

Related: When flying far away from their facilities, they can bring common parts aboard. See What is this orange container labeled “Flight kit wheel”?.

In some cases airlines will follow the "what goes around comes around" principle and will loan or sell parts, and consumables like tires, to another operator that is "AOG" (Aircraft On Ground) to the extent they can without jeopardizing their own operation.
What that means is, in an airline's spares inventory it will have a baseline minimum inventory requirement of components to cover its own AOG situations, things like spare actuators, servos, pumps and other odds and ends, called "AOG stock".  If someone on the airport is hard up and approaches you, and you have the part but it's part of your own AOG stock, you won't lend or sell it because of the risk of your own AOG situation arising before you can replace the AOG unit you loaned out or sold.
If it's part of your larger non-AOG spares inventory, and you can afford to part with it temporarily, you might do so, and you may agree to loan it or sell it.  That's if you operate the same airplanes and have similar parts stocks with the same part numbers.  And airlines that operate the same fleets do buy and sell spares from each other, wheeling and dealing from time to time.
What happens probably more often is the stranded operator will have the required parts shipped "AOG" from either its own spares stock, from an OEM's spares stock from the nearest depot (OEMs will normally maintain spares depots at key locations worldwide) or possibly from the supplier of the part itself (both the OEM and the larger parts suppliers will have their own AOG spares support).  Whoever is fastest (or cheapest if time is less critical).  Often if the part is fairly small, and will fit in a Lear or Citation, the airline (or the OEM if it's something under warranty, and the operator is an important customer) will charter a corporate jet to fly the part to where it's needed (this is done a lot in the corporate world).
In the case of that UPS flight, it's more than just the tire replacements; wheel unit fuse plugs, or the wheels themselves, probably had to be replaced and a number of special inspections had to be done, so even if there were tires on the spot, the airplane was probably stuck there overnight anyway.
For the tires, say they were Goodyears. UPS might have had a set shipped from the nearest Goodyear distributor, which is in Dubai, which for an AOG would have probably would have arrived that evening or overnight.  If there was another airline on the field with the same tires, UPS could have tried to buy a set from them, but they'd have to get lucky and find someone with the same PN tires that were surplus to their own AOG needs and willing to give them up.  Or UPS may have just put a set of tires from their own spares on the next UPS flight going to Delhi.  Or they may have had Boeing's AOG spares take care of it.  They probably would have done whatever was fastest under the circumstances at reasonable cost.
There are also 3rd party spares and logistics companies that some airlines contract to handle all that stuff for them, especially if they are smaller.  Like these guys.

This was very informative. Reason why I thought it could be parts related is they kept with taxiway blocked. For inspections, they could have moved to maintenance bay after fixing flat tires rather than impacting operations of the airport

Ty I was many years in the Regional airline business.  They couldn't move it without some special equipment to set the gear trucks on, if there is such a rig for that airplane, or replacing the wheels, so they were stuck there until the job was done likely.

Reading the AVHearald report, it's likely that they had to order parts in. AVH said all MLG tire fuse-plugs blew. Not likely that UPS would be able to scrounge all the required parts for 16(+?) wheels at one airport.

For Aircraft on Ground (AOG) situations, airlines will send parts and mechanics on flights from maintenance hubs to the AOG aircraft. There are also service engineers for the aircraft manufacturers at just about every airport around the world, so they'll work with the manufacturer's support services to get the AOG aircraft up and flying ASAP. Depending on the tasks, the manufacturers even have teams that are deployable to remote locations that will bring all tooling, parts and mechanics needed to make the aircraft airworthy again.


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Do pilots need to know how to use a sextant?
Can a sextant be used while flying? How accurate/reliable is a sextant, both standing still and at 500+ mph? Are sextants found in any cockpits today?

Not any more, technology has superseded them.  The British "V" bombers all had a sextant which was used to cross check the inertial nav system on ocean crossings when no ground navigation sources were available.

I'd like to see an answer give more attention to the differences between a nautical sextant and an aircraft bubble sextant, and the reasons why a nautical sextant would not work well in an aircraft.  (Possible lack of clear horizon being one-- height of eye can always be corrected for-- maybe it would take a bigger astrodome to give the sextant  a clear view of horizon and star/sun simultaneously-- also a nautical sextant can't be used in the middle of the night!)

Can a sextant be used while flying?

Yes it can. Some aircraft like 747 for example had a sextant port to allow celestial navigation.

How accurate/reliable is a sextant, both standing still and at 500+ mph?

Sextant is not accurate. At least not accurate enough for today's navigation purposes. You cannot perform a RNP-RNAV approach with a sextant. Per wikipedia, the best accuracy you can get with a sextant is between a tenth to a quarter of nautical mile. But it is a very good alternative if everything else fails.

Are sextants found in any cockpits today?

As for sextants in modern cockpits, my best guess is you won't find any, at least in the form of one "left in a drawer". See that image of a sextant being operated on a VC-10. It's more like a submarine periscope than a traditional naval sextant.

Retrieved from airliners.net unknown copyright owner.
Finally, as KRyan notes and as voretaq7 wrote in their answer, pilots don't need to know, but they can attend training in order to know.

That picture is all kinds of awesome

Tenth to quarter of a mile is as good as you can get with any other en-route navigation except GPS. According to the voretaq's answer it is rather a mile or two in practice, but that is still comparable with VOR at longer ranges and completely adequate for en-route navigation. The main problem is that it is tedious.

Ooh, a celestial navigation question...about airplanes... I never thought I'd see one of these in the 21st century!
To answer the basic question: Pilot training doesn't cover sextant use, but there is still a "Flight Navigator" certificate (you'll find it in Part 63 of the FARs), and an associated Flight Navigator Handbook (FAA-H-8083-18), last updated in 2011. The written test question bank is also available if you want an idea of what the FAA expects you to know in order to earn that certificate.

Can a sextant be used while flying?

Yes, if you have a clear view of the sky overhead.
Aircraft used to be designed with "sextant ports" as shown in Stelios' answer, and special aircraft sextants (indirect-sighting, with a bubble to provide a horizon, and usually some kind of averaging mechanism and timer).

How accurate/reliable is a sextant, both standing still and at 500+ mph?

A sextant is a pretty accurate tool under the right conditions, if you know how to use it.
Stationary on the ground you can do a surprisingly good job with a sextant, see for example the US/Canada border monuments. I'm out of practice, but on calm water I used to be able to fix my position to within about 1 minute of arc (1 nautical mile) on any given line.
To put that in perspective, that's slightly better navigational performance than a VOR provides at distance (2-4 nautical miles), but substantially worse performance than GPS (within a few feet).
As far as reliability, the reliability of a ship's sextant is pretty good: much like a mechanical E6B flight computer it'll work fine unless you abuse it (drop, step on, etc.).
Reliability of an aircraft sextant (or any other bubble sextant with an averaging timer) is not quite as good because there are more moving parts involved, but properly maintained they could be trusted to work when you needed them (and most failures could be compensated for by a decent navigator).
The problems with sextants that make them impractical for modern air travel are:

Speed
Sextants don't tell you where you are, they tell you where you were when you made the observation. The averaging mechanisms I mentioned earlier allow aircraft sextant to partially account for the fact that the aircraft was moving (both vertically and horizontally), but at modern speeds you cover a minute of arc pretty quickly. Radio navigation systems and GPS provide much faster position updates than you could get with a sextant.
Complexity of the instrument
Aircraft sextants are precision instruments. They need to be maintained and calibrated or they won't be accurate when you need them, and as I mentioned there are a lot more moving parts to an aircraft bubble sextant than a basic ship's sextant.
Complexity of the process
Using a sextant also require an operator (navigator) who knows what they're doing in order to take the sighting and compute the position, compared to radio navigation (VORs, VOR/DME, or GPS).
Most pilots don't have any training in using a sextant (my experience comes from sailing), so having one would be useless to them.
Weather
While the sextant is a reliable instrument the stars are not reliable: They're always there, but sometimes they're not visible. You can't see most of the stars & constellations you would take a sighting of during the day (usually only one star is visible), and clouds can obscure them when you want to take a sighting, delaying or preventing your position fix.

Are sextants found in any cockpits today?

Certainly not in any light GA cockpit I can think of, and probably not in any single-crewmember cockpit: Even if you held both a pilot certificate and a flight navigator certificate a pilot with their head buried in the sextant is rather useless for actually flying the plane.
I would assume that most commercial cockpits also no longer have a sextant as modern jets don't include a sextant port, so there's no way to take a sighting.
There might be a few restored vintage aircraft that have a working sextant onboard, but I doubt those are used for practical navigation (they're probably not used at all, except possibly to demonstrate how it was done).

Fantastic answer. Re "Are sextants found in any cockpits today?" I seem to recall that one of the US military long range recon. plane  (Blackbird?) still has the system although redundant today due to retrofits of GPS etc. The thing with US military aircraft is that many started their life in the early cold war era which probably meant sextant navigation was imperative on long transoceanic flights. The same models, in substantially, unmodified forms still are flown today.

Not sure about the Blackbird, but it wouldn't surprise me if the B-52 fleet has sextant ports, and if the ports are there the military would probably also keep the sextants available (belt-and-suspenders and all...)

One problem with sextants in-flight is that they measure the angle between a star and the horizon. On ships, the horizon is in (nearly) horizontal direction, in 35000 ft height one may already have to make a minor adjustment to account of for that (at least for maximum precision).

@HagenvonEitzen This is why bubble sextants (which provide a virtual horizon) are used on aircraft. They're also used anywhere else you don't have a good natural horizon (in haze, for surveying, etc.)

The Blackbird had a celestial navigation system, according to one of the pilots speaking at the Udvar Hazy Center, but it wasn't what you'd consider to be a sextant (though it surely worked along the same principles).

Just as a sidenote: there are ways to circumvent bad weather and not being able to see stars the navigator knows. For example Celestron's "SkyAlign" technology only asks the user to pinpoint any three "bright" celestial objects, which an onboard computer then compares to a database (and time + approximate location), and pinpoints the user's location. Technologies like this could work well for airplanes too, if the airspeed and altitude instruments could input more data for calculating the position.

@V-J That still requires a view of the sky sufficient to pinpoint three bright celestial objects (impossible under an overcast).

@voretaq7 Overcast would make it rather impossible for visible spectrum, yes. Although, the same technique could be used for other spectra such as microwaves or infrared, which could pass through the clouds to detect stars, but at that point your state of the art detection system might as well use any other easier way of locating the plane, or just keep flying.

plenty of DC-3s with nav bubbles

As far as the point about sextants telling you where you were when you made the observation, it would be feasible to use sextants to update current position and let the flight computer use dead reckoning or IRU to update.   Like you said, a sextant is better than VOR/DME for position updates, which when GPS is blocked is still the standard way to compensate for IRU drift.

As a retired USAF Navigator, I can tell you C-130's stopped using celestial Navigation by the mid-1990's.  As far as accuracy, if you were good at it, and that took years of practice, you could get within a couple of NM's, especially at night when you can use three stars.

Yes they can use but no its almost never used anymore and its not a requirement for the PPL or any other license above that here in the US as far as I know. There was at once point a "Flight Navigator Rating" or something to that effect that required knowledge of sextant use.

Addressing the first (headline) part of the question; Do pilots need to know how to use a sextant? I can think of one circumstance where it might come in handy - when you've ditched and are in a raft!
As an aside on sextant accuracy; originally, mariners were sailing from Southampton and trying to find New York. Getting to within a mile of the entrance to the Hudson after sailing 3,000 miles is accurate enough.

Funny you mention 3,000 miles, that's the exact distance that a sextant will give for the distance of the sun from the earth... but there is most certainly some kind of 'rational/scientific reasoning/mathmatical explanation' for that . (*Of wich, I, personally cannot provide at this point in time).

Errr, no it's not. A sextant measures angles. It is not used, and cannot be used to measure the distance of celestial objects. . You have made a bizarre (and easily disproved) claim with no attribution. Extraordinary claims require extraordinary proof.

@Simon He was going to write the proof in the margin, but there wasn't enough room...

Yes, but it's not that common, except for a few, rare military applications.
The Lockheed A-12 and SR-71 aircraft featured a Nortronics NAS14-V2 Astralinertial navigation system which used a self controlled sextant combined with an internal navigation system to locate and track the aircraft's position in space and could be coupled with the AFCS to provide steering cues.  The system provided excellent accuracy for the day and could track stars both day and night.  It had additional advantages in that it was totally independed to terrestrial or space based navigational aids to locate the aircraft, increasing stealthiness and reducing vulnerability.
Some long range commercial airliners installed astral navigation systems as well during the 1960s and 1970s.  But with modern INS and GNSS equipment, it went the way of the dodo along with systems like NDB, LORAN, etc.


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Why do the engines of the PAK FA (Sukhoi T-50) seem not to be not aligned parallel to each other?
The nozzles of the Sukhoi T-50 seem to be aligned not perfectly parallel. However, it is not very pronounced.
What may be the reasons for aligning the nozzles engines not perfectly parallel.
Edit: in my original post I asked about the nozzles, however, as it was pointed out, this plane used 3D thrust vectoring, so the nozzle arrangement might be this way by chance. However, also the engines seem to be aligned not perfectly parallel. The question has been edited accordingly.

Image source Wikimedia
Edit 2: Here is another picture
Image source, Wikipedia again

Might be the thrust vectoring being used.

You might clarify if you're actually talking about the nozzles, in which case Adder's answer applies. But looking at aircraft photos I think you are probably asking about why the entire engines are skewed slightly apart.

Good point about the engines. Initially it was the alignment of the engines that triggered this question. However, it got formulated to be about the nozzles, which is not the same for 3D thrust vectoring aircraft.

Regarding your edit, this might just be the paint job playing tricks on you.

@sanchises: I am not sure if it is solely an illusion. Compared with other planes, e.g. the Mig-29 the engines of the Pak-FA seem not quite parallel.

The Sukhoi T-50 operates Thrust vectoring nozzle control at low speeds. The nozzles are a part of the Fly By Wire system and will deflect as per the control input given by the pilot. The nozzles move in coordination with other flight controls and it is not necessary for them to be parallel in all conditions of flight. However on the ground they will appear to be parallel.

To add to this, it's actually a condition of the PAK-FA nozzles that they're not parallel. Otherwise they wouldn't be able to perform 3D thrust vectoring (as they'd lack moment in 2 of the axes).


text:
How to design a near vertical take off airplane?
How to design a near vertical take off airplane, conceptually?
Say I have an airplane, similar to a Zenith 701. Say that it has a stall speed of 50 mph and a wing area of 60 sq ft, and it weighs 500lbs.
Now I want to make it land at 20 mph with full power and my tail volume is sufficient to keep the nose up at 30 deg. I also have slats and flaps to ensure my wing stalls above 30 deg AOA.
Say my max HP is 100.  I understand, as a general rule for GA aircraft, thrust is 4x100hp=400lbs. Vertical thrust would be 400 lbs x sin 30 deg x 65% prop efficiency for a prop in climb, giving me a vertical thrust component of 400 x .6 x 65% = 160lbs vertical thrust.
Now, I only need to produce 500-160=340 lbs of lift from my wings. So I increase my wing size so that I can fly at 20mph with a lift of 340 lbs.
Is this correct, at least conceptually?

Yes that is conceptually correct. You might have to mount a huge area, since lift is proportional to $V^2$.
Issues to resolve that I can see:

The craft would be relying upon aerodynamic forces to maintain attitude, which will be very ineffective at 20 mph. A secondary pressure relief valve system for attitude control would be advisable.
Landing at 30 degree nose up will be not easy from ground view perspective, and for landing gear design.
Ground effect will change the attitude of the craft if not carefully designed.
A wing that does not stall until 30° will be a tall order, and landing at the brink of stall is not advisable, you'll need some margin.

And if you do get to the stall AOA on a slatted wing, the stall can be quite violent, basically a LE stall across the entire wing all at once.  I know a guy who was able to coax a stall from a Pegazair with the slats out, by accelerating it with hard inputs, and it flipped so violently his flight bag came out of the baggage area and ended up on his glareshield.

What is a "secondary pressure relief valve system for attitude control"???

FYI: The zeniths don't stall until past 30 deg.  I've heard some of the highlander bush pilots  do high alpha landings: drag the tail on the ground first, then slam the wheels down , up to 1,000ft/min with their 36" suspension......!!

Also, the lazairs stall at 18mph.  I guess you could just copy their control volumes to get the right sized controls.

A separate pressure relief valve system: a couple of pipes running to front & back & wingtips, if required a valve opens and lets go of controlling airflow. Like in the Harrier.

oh. nice. Why the wing tips? I assume that's for the wingtip ailerons and the prop wash takes care of the tail controls? I'd say, go for flaperons, full length flaps and ailerons. Better yet, a junkers flap, which sits below the wing. This is what the Zenith 701's have for low speed maneuvering, which I understand can take off in 25' with headwind, or 50', no headwind.  Zenith701's also have extended wing roots. Their wings don't start at the fuselage. This is give more clear air to the tail in low speed flight I'm told.

Hmm, after a little deliberation, I don't think the high alpha route is the way to go. If it were, the Alaskan bush pilots would be doing it and they can land in 10' and take off in 10' with just a normal looking landings and takeoffs.  A 10' landing and 10' takeoff is good enough for me!!!

@Fred I assume he is talking about something similar to how a Harrier maneuvers when hovering.  It uses bleed air from the engine to control movement around all three axes.  Of course without a jet engine I am not sure how you would pull this off.

The way to do that without a jet is distributed propulsion, so that 100% of your of your wing span, tail and all controls are is prop wash.


text:
What happens when you put vortex generators on both sides of a symmetrical vertical tail?
What happens when you put vortex generators (VGs) on both sides of a symmetrical vertical tail?
I understand a horizontal tail with a classic foil like a NACA 0012 has a max cl of about 1.5.
I understand VGs add about 0.6cl.
What happens when you put VGs on both sides of the vertical tail?
For sure it will allow lower speed or better control at lower speeds, but will it actually bump the max cl from 1.5 to 2.1 at positive and negative angles of attack?
That's a 40 % increase in performance!!!
So.... can I decrease my vertical tail by roughly 40% and still get a similar EFFECTIVE vertical tail volume ( vertical tail moment)?

Just because VGs can delay the flow separation (with no guarantee of success), it doesn't mean you have increased your tail volume. In fact, you have the exact same tail volume unless your vertical tail is separating at small sideslip, which probably indicates bad design or bad manufacturing.
If the VGs do increase the max Cl, it means you may be able get to a higher slip angle, perhaps more crosswind capability if your rudder is up to the task.

I'm not sure adding Vg's to both sides of a vertical tail will increase the cL at all since  you're effecting both sides at the same time, and not increasing differential pressure or increasing the velocity of only one side, no?

@Fred VGs do not modify the camber, so I don't see how it can increase the lift slope. VGs serve to energize the boundary layer and delay flow separation.

I understand Vg's can act the same as a slat on a wing with a NACA2514 foil or similar. The Zenith 701 uses a slat. A copy of the Zenith 701, the Savanah, uses vg's instead of slats for less drag and higher cruising speeds, or do vg's just allow for a higher AoA ( therefore higher cL)?

@Fred I don't have the wind tunnel data or the flight test reports, so I can't surmise on the particular design. However, if there's an area of separation on the wing, especially on flaps down at nominal AOAs, VGs have the potential to reattach the flow and decrease the overall drag.

@ jimmy Vg's do allow for a higher AOA, and therefore a higher climax, as per the Zenith design school: http://www.zenithair.com/stolch801/design/design.html. Vg's don't change the slope of the lift slope, but rather extends it.

@Fred No one is disputing that VGs have the potential to extend CLmax (not guaranteed). However, what does this comment have to do with the answer or your original question? I think I've explained why CLmax is not a particularly useful measure for a V-tail.

It doesn't work that way on vertical tail surfaces.  There aren't really issues with LE stall on rudders that you need to delay.
My old '68 Cardinal had a STOL kit that added VGs to the vertical fin.  They aren't mounted at the LE like a wing, to delay separation at the LE, they are installed just forward of the hinge line to keep flow attached to the rudder itself over a wider range of movement than without.  What you got from it was a bit more rudder effectiveness at the lower speed the plane was capable of because of the wing mods.  It doesn't allow you to make the surface substantially smaller, which will degrade your yaw stability.
You see that on most airplanes where VGs are added to vertical surfaces.  They are almost always just forward of the hinge line.
If you really want a much smaller vertical surface, you can do an all-flying surface like Zenith, but you still need substantial fixed side area for passive weathervaning.  The Zenith does this with large slab sided fuselages.
However you could get good natural passive weathervaning with an all flying surface by using an anti-servo tab on its trailing edge.

How does adding a anti-servo tab on a all flying horizontal elevator add stability? I assume it acts as a slatted flap and adds somewhere between 0.7 and 1.3 cL.

An anti servo tab works to aerodynamically fix a stabilator surface at a given incidence by creating a counteracting force to pitching moments in the surface and results in stick free stability like having a fixed stabilizer. Trailing edge goes up, anti-servo tab goes up more, providing a counteracting moment.  At a certain spot it's all balanceed.The sizing and gearing of the anti-servo tab to the movement of the stabilator determines the overall pitch feel force characteristics,and the gearing can be mechanically biased to change the speed at which equilibrium is found,giving you pitch trim.

If you put an anti-servo tab on an all flying rudder like a Zenith's, it would just be centered with the rudder centered and if the airplane yaws the rudder will want trail, displacing it from neutral, and the displacement makes the anti-servo tab move in the same direction, tending to drive the rudder back to neutral, and this makes the all-flying rudder tend to act as a fixed fin.  Not sure how well it works in practice, although in pitch they are quite effective.


text:
What are typical stability analysis dampening values?
I'm not an aerospace engineer, so don't know if I'm asking this correctly. I've been using the new XFLR5 with stability analysis. Just don't know what values are good.
What are typical stability analysis dampening values for GA aircraft
for:
Lateral modes:

roll control
dutch 1
dutch 2
spin

Longitudinal modes:

short period 1
short period 2
phugoid 1
phugoid 2

For the eigenvalues:

The Roll Subsidience should be negative.
The Dutch Roll values should have negative real parts, and they will be complex conjugates unless something is really wrong with your model.
The Spin (spiral) mode should ideally have a negative sign but there is some leeway to it being positive if the modulus is small.
The Longitudinal modes should both have negative real parts, especially the short period mode (the one with the highest frequency, in the 10Hz range where the pilot cannot react in time to damp it)

Assuming the eigenmodes are convergent, the dampening coefficients need to be large enough to kill any oscillation in a reasonable time frame. What is reasonable is often up to the particular design case; for example fast enough to prevent resonance with some structural mode, or far away from the pilot´s natural feedback loop to avoid PIOs .
For a brief primer on stability, see here: https://courses.cit.cornell.edu/mae5070/DynamicStability.pdf


text:
What is the difference between experimental and amateur built aircraft in the US?
What is the difference between experimental amateur and amateur built aircraft in the US, or are they the same?
If not, is there a difference re specifications, inspections, engines, passengers, etc.?

Amateur built are always experimental, but experimental is not always amateur built. Other than that I don't think there are many differences. Are you asking in terms of inspections? Qualifications to fly? Limitations on flight? There are a number of "experimental" categories in FAA land (like experimental-amateur built or experimental-exhibition only)...

The FAA provides pretty clear definitions and/or explanations:
Amateur built:

Title 14, Code of Federal Regulations (14 CFR), part 21, section
  21.191(g), defines an amateur-built aircraft as an aircraft "the major portion of which has been fabricated and assembled by person(s) who
  undertook the construction project solely for their own education or
  recreation."

They define Experimental as a category, since it has to do with how the airworthiness cert is issued:

A special airworthiness certificate in the experimental category is
  issued to operate an aircraft that does not have a type certificate or
  does not conform to its type certificate and is in a condition for
  safe operation.  Additionally, this certificate is issued to operate a
  primary category kit-built aircraft that was assembled without the
  supervision and quality control of the production certificate holder.

And further

Special airworthiness certificates may be issued in the experimental
  category for:

Operating amateur-built aircraft: to operate an
  amateur-built aircraft in which the major portion has been fabricated
  and assembled by persons for their own recreation or education.
Operating kit-built aircraft:  to operate a primary category aircraft
  that was assembled by a person from a kit manufactured by the holder
  of a production certificate for that kit, without the supervision and
  quality control of the production certificate holder.

So... All amateur built aircraft are experimental (flown under an experimental airworthy type certificate) but not all experimental's are amateur built, for example a pre-production aircraft may need to be registered as an experimental for test flights before a type certificate is received.

"Experimental" was originally applied to development test aircraft only.  The application of the term to amateur builts is an artifact of the early days of homebuilding in the early 50s, when the Civil Aviation Authority (FAA's predecessor) was persuaded to create a formal licensing structure for amateur built airplanes created for personal use, and they decided to piggyback on the existing Experimental category normally used for test aircraft, as the path of least bureaucratic effort.
So you have a somewhat odd categorization, done nowhere else, where airplanes NOT created for development and testing, but just built for people to have fun and fly, still have EXPERIMENTAL plastered on the side.
In Canada the homebuilt category was originally called "Ultralight", but then in the 70s actual ultralights appeared on the scene and the name wasn't really suited to 180 mph airplanes like Thorp T-18s, so a new Amateur Built catagory was created to cover traditional homebuilts, with ultralights just ultralights.

As I understand it (I am not a pilot, nor do I live in the US) in the US:
"amateur built" is a subset of "experimental".
"experimental" aircraft are assesed and issued with airworthyness certificates on an individual basis under requirements less rigorous than the normal type-certification process. However the FAA doesn't want manufacturers using experimental certification as a way to do an end-run round the normal certification process, so they put limits on what aircraft can be registered as "experimental".
"amateur built" is a subset of experimental for aircraft that are more than 51% built by amateurs and is one of the least restrictive experimental categories.
Another subcategory of experimental that the aviation hobbyist may run into is "experimental exhibition", this category is often used for ex-warbirds and is also sometimes used as a means to heavily customize aircraft. The downside as I understand it is you have to convince the FAA that you will genuinely be using the aircraft for exhibition purposes.
Regarding passengers as I understand it no experimental aircraft is allowed to carry passengers for hire and some are not allowed to carry passengers at all. As I understand it amateur builts are allowed to carry passengers not for hire after passing some additional testing/proving.  I don't know if the rules on carrying passengers are different for other types of experimental.


text:
How did the propellers attach on a Lazair attach if the shafts had a woodruff key instead of opposite thread?
How did the propellers attach on a Lazair attach if the shafts had a woodruff key instead of opposite thread?
I understand the early Lazair prop shafts had a woodruff key.  How were
the props attached?
Don't you need opposite thread?
I assume you couldn't just use a cotter pin and a washer, as the props would try to shear off the cotter pins.

I believe that while the original engine shafts had a slot for a woodruff key, no key was actually used in attaching the props to the crankshaft. Instead, a nominal arrangement of multiple (6, I believe) bolts attached the props to the crankshaft for mutual transmisal of both engine torque and prop thrust. Presumably the bolts were secured with safety wire, but I have no data on this.

It's not good practice to use a woodruff key to transmit thrust forces on a rotating shaft. Ideally you'd use a woodruff key to transmit the engine torque to the propeller, and use a separate clamp nut or series of bolts running through the crankshaft hub and the prop hub to prevent the propeller thrust from pulling the prop off the end of the engine crankshaft.

So... how did they attach the prop hub( if there was one)?  I'm thinking only 1 bolt at the end of a threaded hollow shaft... I think the standard practice is to thread a prob hub onto a threaded shaft with opposite thread, and then use bolts to attached the prop blades.